What was being mourned in lines 2199-3182 of Beowulf?
At the beginning of this section of the epic poem, Hygelac is dead and people are mourning him.
But I think what you are probably thinking of is the part quite a bit later on where Beowulf himself dies. Beowulf dies while fighting a dragon. He has to fight the dragon because it has burned down one of his halls while trying to find someone who had stolen something from its hoard of treasure.
Beowulf goes and fights the dragon but is burned to death. He is able to draw some blood from the dragon, but is not able to withstand its fire.
I just reread the parts and I believe that Beowulf is actually mourning Grendel. Grendel has been a worthy adversary and Beowulf lives for the adventure of fighting worthy foes. He places Grendel's head on a stick for all to see his victory, but he returns telling the story of the battle's end and how he had finished off Grendel.
The other though is that Beowulf is mourning his own youth. He had fought many brave battles including the battle with Grendal, the most worthy of opponents, but as he has aged he does not have the same vigor and strength he once had. Ths in time leads to his death after he fights the dragon and loses.